This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

The Retina

The retina consists of many layers of nerve cells. It is like a peripheral brain that does sophisticated processing on the visual image before passing information on to the brain. Light passes through the blood vessels and several layers of nerve cells before arriving at the photoreceptors, the rods and cones. The rods and cones are receptor cells of the eye. As you can see from the diagram, the names are appropriate. Cones are the cone-shaped cells at the bottom; rods are the rod-shaped cells next to them. The diagram is highly simplified, showing only the cell types and not their density. Millions of cells are packed into the retina.

Of what does the retina consist?

Simplified cross-section of the retina. Light enters from above.

How do the rods and cones work?

Rods and cones consist of layer upon layer of folded cell membranes. In the following diagram, a tiny segment of a rod receptor is magnified to show the layers. Each layer contains light-sensitive chemicals. When light strikes the chemicals, it provokes a chemical reaction, generating a tiny electrochemical current that influences nerve cells near the receptors.

Inside rods and cones are layers of folded membrane containing light-sensitive chemicals.

What are the eye's chromatic and achromatic receptors?

Although rods and cones both use layers of chemicals that react to light, they use different chemicals, so they respond differently to light. Cones are specialized for color vision. They are called chromatic receptors, the root chroma- meaning color. Rods, by contrast, are specialized for black-and-white vision. They are called achromatic (no color) receptors.

What are two other differences between rod and cone vision?

There are two other important differences between the rods and cones: (1) rods have greater sensitivity to light and (2) cones have greater acuity (ability to pick out details). Because rods are more sensitive to light, they are the receptors we use at night. When you see objects outdoors by the light of the moon, there is not enough light to activate the cones, so you use primarily rod vision. This is what gives the nighttime world a black-and-white (achromatic) appearance. However, our acuity is not as good at night. We cannot see fine details as easily, because the cone receptors are not as active in the darkness.

How long does it take for rods to reach full dark adaptation?

When light is not present, the rods and cones build up their supplies of photosensitive chemicals-a process called dark adaptation. This makes our eyes much more sensitive to light. The rod receptors become more and more light sensitive for 30 minutes after being deprived of high light levels. When rods are fully dark adapted, they are 10,000 times more sensitive than they are when they are bleached by strong light. This is why dark adaptation has such a noticeable effect in places like movie theaters. People coming in from bright daylight stumble around in darkness, while people who have been sitting in the theater for a long time can see clearly, because their eyes are adjusted to the darkness and more sensitive to light.

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